Because of mechanical considerations, it is not always possible to use the most corrosion resistant aluminum alloy for a specific application. The availability of coated alloys has gone a long way toward solving this problem. More specifically, chromate conversion coated aluminum alloys have long been used to improve aluminum alloys' corrosion resistance. In the prior art, the alloys were treated with a chromated deoxidizer to etch away the natural and heat-treat/oxide and then coated in the conversion process with a corrosion resistant coating.
The 7000 series aluminum alloys are aluminum/zinc/magnesium alloy and aluminum/zinc/copper alloy systems. The 7075 high-strength aircraft alloy that contains aluminum, zinc, magnesium, and copper has been used for rocket motors. The 7075 alloy is heat-treated to avoid stress corrosion cracking, an extremely serious, very difficult to control mode of corrosion failure. The treatment is known as overaging (a longer heat treatment beyond that ordinarily utilized to obtain maximum strength) in which copper-containing components of the alloy are dispersed as secondary precipitates throughout the bulk of the alloy. The alloy produced is called 7075-T73 or 7175-T-7452, the latter specified for hand forgings of a somewhat higher-purity grade of 7075. Although stress corrosion cracking is significantly reduced by overaging, pitting corrosion is exacerbated. Thus, there is a need for an improved method or formula of chromate coating.
When coated aluminum alloys are manufactured for use in military applications for which high strength characteristics are a primary requirement, a stringent corrosion resistant test must be met whereby the finished alloy is subjected to a salt spray for a lengthy period of time. Prior to the present invention, it was extremely difficult to pass the salt spray requirement imposed by the military for aluminum alloy hardware. Most of the anti-corrosion, chromate conversion type coatings used on overaged heat treated 7000 series aluminum alloys have been very susceptible to localized salt spray corrosion, i.e. pitting.
The process of coating aluminum alloys actually is a chemical reaction involving two reactants: the chemical solution and the metal substrate. The nature of currently used alloys is such that the substrate is adverse to reacting uniformly with the solution. This is one of the main obstacles to overcome in achieving corrosion resistant alloys of this type.
U.S. Pat. No. 4,131,489 issued to Newhard, Jr. on Dec. 26, 1978 relates to a method of improving both corrosion resistance and paint adhesion of chromate conversion coatings on aluminum and alloys thereof (lines 3-6, 17-20 and 31-34 of col. 2). The conversion coating solution comprises chromate ion, phosphate ion and fluoride ion and during the coating process, the free fluoride ion content is maintained within the desired limits by adding fluoride in the form of hydrofluoric acid to the coating composition (lines 33-36 of col. 3). The salt spray test is governed by ASTM B-117, Standard Method of Salt Spray (Fog) Testing which applies to all corrosion testing referred to herein. The preparative method includes contacting the aluminum surfaces to be treated with the coating composition for a time and at a temperature sufficient to produce an effective coating (lines 34-37 of col. 4). As stated in lines 56-65 of col. 4, the conventional coating process includes the steps of rinsing the metal following each immersion step. This reference relates to the use of free fluoride, preferably in the form of hydrofluoric acid, in the chromate conversion coating composition for improving both the corrosion resistance and paint adhesion on aluminum alloy.
U.S. Pat. No. 4,451,304 issued to Batiuk on May 29, 1984 discloses a method of improving the corrosion resistance of chemical conversion coated aluminum, especially to meet the salt spray test requirements imposed by the military (lines 15-20 of col. 1 and lines 48-51 of col. 3). The method comprises the following steps sequentially: vapor degreasing, alkaline cleaning, rinsing with water, deoxidizing with a chromated or non-chromated deoxidizer, rinsing with water, exposing to a sodium nitrite solution, rinsing with water, chemical corrosion coating and finally drying the aluminum (lines 13-25 and 37-40 of col. 4). The preferred deoxidizer includes fluoride ion obtained from hydrofluoric acid or any suitable salt (lines 47-48 of col. 4). The chemical conversion coating solution may be Alodine 1200S.TM. (manufactured by Amchem Products, Inc.) which contains sodium fluoride (lines 11-13 and 24-25 of col. 5). This reference relates to the use of free fluoride in the deoxidizer and use of fluoride in the form of sodium fluoride in the conversion coating solution utilized in the process for corrosion resistance coating of aluminum alloy.
Chromate conversion coatings on aluminum surfaces is disclosed in U.S. Pat. No. 4,531,978 issued to Otrhalek et al. on Jul. 30, 1985 wherein the fluoride sources include hydrofluoric acid, sodium fluoride, ammonium bifluoride and others (lines 34-42 of col. 2).
U.S. Pat. No. 4,036,667 issued to Simon on Jul. 19, 1977 relates to chromate conversion coating process for aluminum and its alloys and wherein the source for fluoride ion includes sodium fluoride or potassium fluoride.
Newell et al. disclose in U.S. Pat. No. 3,752,707 issued on Aug. 14, 1973, a method for corrosion resistance coating on aluminum alloys using solution containing chromium compound, rare earth salt and inorganic fluoride (from NaF, HF, etc.).
U.S. Pat. No. 4,146,410 issued to Reinhold on Mar. 27, 1979 teaches non-ferricyanide chromate conversion coating for aluminum surfaces with enhanced anti-corrosion and improved paint bonding characteristics.
A further impediment to the formation of continuous coatings that must withstand corrosion occurs when it becomes necessary to apply the conversion coating by brush in the case where it is not possible to apply the chromate coatings by the usual immersion techniques. This will occur, for example on a missile motor when a bearing ring or other high-strength aluminum alloy attachment is connected permanently to the motor body, and any contact of the chromate solution with the body will cause deleterious chemical interactions. Thus, the use of a brush or other hand applicators is mandated for this as well as brush-on formation of coatings that must in addition be repaired because of physical damage to the coating.